The invention is concerned with electrically driven locks, and in particular is directed to solenoid driven deadbolt locks, a particular case being those driven by keep solenoids.
Electrically operated locks are often driven by a solenoid, with the reciprocable solenoid plunger driving a deadbolt or spring latch of the lock. In such a lock a spring can be positioned to push the deadbolt or spring latch to the extended position, while retraction is effected by powering the solenoid, to retract the plunger.
Electric locks have often employed a specific type of solenoid known as a keep solenoid, so that, especially in the case of a deadbolt lock, whether the lock bolt is extended or retracted, no power is required to maintain the position. A keep solenoid has both a permanent magnet and an electric coil. The spring loaded solenoid plunger is released outwardly when the coil is energized in a first polarity, creating an approximately equal but opposite magnetic force to that of the permanent magnet, effectively neutralizing the permanent magnet. This allows the spring to cause the outward movement of the plunger. When the solenoid coil is energized in the opposite, second polarity, it creates a magnetic force aligned with that of the permanent magnet, essentially doubling the magnet pulling power, overcoming the spring and pulling the plunger to the inward retracted position.
Thus, use of a keep solenoid in an electrically actuated lock is advantageous, due to the fact that the locking element can be moved from lock to unlock and from unlock to lock states with momentary use of power, with no additional power required to keep the lock in either the locked or unlocked state after the use of momentary power.
However, certain disadvantages arise from the use of a keep solenoid. First, the travel distance of the plunger is limited by the size of the magnet and coils, and the power source. Utilizing commercially available battery cells such as AA, AAA or 9 volt, only a small throw distance can be achieved for the lock element, approximately 7 mm or ¼ inch given practical considerations. Theoretically the throw could be longer from action of the spring, but if the throw is too long the solenoid and permanent magnet will not be able to retract the plunger and the lock element. This factor, along with battery and magnet requirements, must be balanced against desired throw distance in the lock design, and thus there is a practical limit on bolt throw distance.
A second important shortcoming of a keep solenoid-driven lock is that the locking element or bolt can be pushed back if someone can gain access to any part of the bolt and push it back using a finger or a tool. This is also a failing of simple solenoid driven locks, as opposed to keep solenoids, as outlined above. One need only overcome the relatively light force of a spring in order to push the locking element back and open the door or cabinet.